Motorized adjustable convergence mechanism for projection displays

ABSTRACT

A light engine for use in a projector, comprising a motorized adjustable convergence mechanism for fine adjusting the relative positions of red, green and blue channel sub-assemblies. Motors are mounted on fixed elements of the sub-assemblies for driving movable elements thereof, including imaging devices thereof such as digital micromirror devices.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to digital projectors incorporatinglight engines with color splitting-converging prisms, and moreparticularly to a motorized adjustable convergence mechanism foradjusting the position of Digital Micromirror Device (DMD)sub-assemblies of a digital projector.

2. Description of the Related Art

A typical color digital projector consists of a lamp, an illuminationsystem, and a color splitting-recombining light engine. The opticalfunction of a light engine is to split uniform illumination light intoRed/Green/Blue (ROB) channels, merging the three channels onto animaging device or optical panel such as an LCD (Liquid Crystal Display)or DMD (Digital Micromirror Device), and then re-combining all threechannels into a single illumination light beam that is projected on ascreen via a projection lens.

The DMD is an electromechanical device consisting of millions ofmicroscopic mirrors that modulates light by independently flipping eachmirror through a +−12 degree angle. The design of the DMD requirescreation of a light cone with an f/#2.4-2.5 for maximum light throughputin the projector. Using three such DMDs on a prism (e.g. Phillips®prism, plumbicon, etc.), a white light cone of f/2.4 can be separatedinto red, green, and blue. Each color is individually modulated by theDMD and then recombined by the prism.

The positions of red, green and blue DMDs are critical for correctcolour convergence of the output image. For an optimal image, the samepixels from each of the red, green and blue DMDs must be 100%overlapped. In normal usage environments, convergence may drift (in theorder of micrometers) as a result of environmental changes, vibration,etc., which can create unacceptable color separation. However, the lightengines used in conventional projectors are located deep inside theprojector and therefore hard to access, making convergence correction adifficult and time-consuming operation for service personnel in thefield.

Accordingly, it is an objective of the present invention to provide amechanism for convergence adjustment without requiring manual adjustmentof the light engine.

SUMMARY OF THE INVENTION

Therefore, according to an aspect of the present invention, a motorizedadjustable convergence mechanism is provided for adjusting the positionof at least two DMD sub-assemblies of the light engine in a digitalprojector.

These together with other aspects and advantages that will besubsequently apparent reside in the details of construction andoperation as more fully hereinafter described and claimed, referencebeing had to the accompanying drawings forming a part hereof, whereinlike numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a digital projector, according to the prior art.

FIG. 2 is a schematic representation of a light engine with plumbiconprism and DMD, according to the prior art.

FIG. 3 is a perspective view of a light engine with motorizedconvergence mechanisms on two channels, according to an exemplaryembodiment.

FIG. 4 is an exploded view of a single channel of the light engine inFIG. 3.

FIG. 5 is rear elevation view of the single channel shown in FIG. 4, andFIG. 6 is a partial fragmentary view thereof.

FIG. 7A is a cross-section view of a light engine according to the priorart, and FIG. 7B is a cross-section view of a light engine according tothe exemplary embodiment of FIGS. 3-6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a typical projector comprising a Xenon lamp and parabolicreflector (1) for creating a light cone (3) that passes through a UVfilter (4) into an illumination system (2), including an integrator rodand lenses for telocentric illumination. A light engine (5) includes acolor splitting-converging prism (typically a plumbicon prism) withthree prism elements and respective red channel (9), green channel (8)and blue channel sub-assemblies (7), each of which includes an imagingdevice (10), as shown in FIG. 2. The prism elements contain dichroiccoatings (6) to separate the incoming white light into blue, green, andred. Each color is then separately modulated at each imaging device(10), such as a DMD. According to the exemplary embodiment, the imagingdevices (10) are DMDs. Accordingly, further reference to imaging devices(10) have been replaced by reference to DMDs, As discussed above, a DMD(10) is an electromechanical device that typically consists of millionsof microscopic mirrors for modulating light by independently flippingeach mirror through +−12 degree angle. Each DMD (10) reflects themodulated light, which is re-converged (11) by the prism and projectedby a projection lens onto a screen to produce an image.

According to the exemplary embodiment, motorized convergence mechanismsare provided for field alignment of the colour channels in the eventthat the unit develops misconvergence of color. More particularly, asshown in FIG. 3, a pair of motorized mechanisms is provided foradjusting the position of two of the three channels of light engine (5)such that two channels are adjustable with reference to the third. Inthe exemplary embodiment, the red channel subassembly (9) remains fixedwhereas the green channel subassembly (8) and blue channel subassembly(7) are provided with motorized convergence mechanisms (12).

As shown best in FIG. 4, each of the blue (7) and green (8) channelassemblies includes movable elements and fixed elements, with motors(13) mounted on the fixed elements for driving the movable elements. Themotors (13) may include, but are not limited to, high precision steppermotors or piezo-actuators. Sensors can also be used to provided positionfeedback.

The fixed elements include a DMD aperture and front air cooling channel(14), DMD and PCB assembly (15), socket mounting plate (16) and fixedstage frame (17). The socket mounting plate (16) is mounted to the prism(not shown in FIGS. 3-6). A pair of vertical adjustment motors (13V),and a horizontal adjustment motor (13H) are mounted to the socketmounting plate (16) for adjusting the position of a floating plate (18)that houses the DMD (10) and associated components.

As shown in FIGS. 5 and 6, the floating plate (18) includes leaf springs(19) that contact inner surfaces of the fixed stage frame (17) forminimizing backlash. Motor shafts (20) are threaded and pass thoughholes in the fixed stage frame and thence into threaded holes in thefloating plate (18). Each motor shaft (20) includes a pair ofdifferential threads. A first thread (e.g. M2.5×0.45) is threaded intothe floating plate (18) while the other thread (e.g. M3×0.5) is matedwith a thread compensator (21) that constantly pulls the fixed stageframe (17) against leaf springs (19). The two threads are preferably ofthe same thread type (right hand or left hand) but have differentpitches, such that turning the shaft (20) one revolution results inadjustment of the relative position between the fixed stage frame (17)and movable floating plate (18) according to the difference of pitches(e.g. 0.5-0.45=0.05), as described for example in U.S. Pat. No. 5,596,404, issued Jan. 21, 1997,

In order to adjust the orientation of DMD (10), a technician actuatesbuttons on a user interface (UI) or remote device, for providing inputsignals to control the motors (13). For vertical translation, motors(13V) are activated to move the floating plate (18) upwardly ordownwardly whereas for horizontal translation, motor (13H) is activatedto move the floating plate (18) left or right. In order to rotate thefloating plate (18), motors (13V) are activated in opposite directions.

An additional advantage of the configuration depicted in FIGS. 3-6, isthat additional space is provided, as shown in FIG. 7B, for cooling onthe front side of the DMD (10), than prior art designs, as shown in FIG.7A. Consequently, the DMD (10) and prism are less sensitive to thermalchanges than prior art designs. In particular, compared to theconventional light engine design of FIG. 7A, the socket mounting plate(16) is located further from the prism, on the other side of the PCBassembly (15), such that the space between the PCB assembly (15) andprism is greater thereby allowing more space for the air coolingchannel.

The many features and advantages of the invention are apparent from thedetailed specification and, thus, it is intended by the appended claimsto cover all such features and advantages. For example, although theexemplary embodiment relates to a convergence mechanism for DMDs, theprinciples set forth herein apply equally to LCD and LCoS, and othersimilar technologies. Further, since numerous modifications and changeswill readily occur to those skilled in the art, it is not desired tolimit the invention to the exact construction and operation illustratedand described, and accordingly all suitable modifications andequivalents may be resorted to, falling within the scope of the claims.

1. A light engine for use in a projector, comprising: a prism forreceiving and separating white light into blue, green, and red light; aplurality of channel subassemblies having imaging devices for receiving,modulating and reflecting said red, green and blue light back to saidprism, which in response re-converges said light for projection onto ascreen to produce an image; at least two of said channel subassembliesbeing provided with motorized convergence mechanisms for adjusting theorientation of respective ones of said imaging devices for convergencecorrection of the image projected on said screen.
 2. The light engine ofclaim 1, wherein two of said channel subassemblies being provided withmotorized mechanisms for adjusting the orientation of a respective twoof said imaging devices relative to a third one of said channelsubassemblies.
 3. The light engine of claim 2, wherein said modulatingsaid two channel subassemblies are green and blue channel subassemblies.4. The light engine of claim 1, wherein said imaging devices are digitalmicromirror devices.
 5. The light engine of claim 1, wherein each ofsaid channel subassemblies includes a plurality of fixed elements, atleast one movable element housing a respective one of said imagingdevices, and a plurality of motors mounted to said fixed elements fordriving said at least one movable element.
 6. The light engine of claim5, wherein said motors are high precision stepper motors.
 7. The lightengine of claim 5, wherein said motors are piezo-actuators
 8. The lightengine of claim 5, wherein said at least one movable element is afloating plate and said fixed elements include a mounting plate mountedto the prism and supporting each respective one of said motors and afixed stage frame mounted to said socket mounting plate andcircumscribing said floating plate.
 9. The light engine of claim 8,wherein said motors include a pair of vertical adjustment motors forvertical and rotational adjustment of said floating plate, and ahorizontal adjustment motor for horizontal adjustment of said floatingplate.
 10. The light engine of claim 9, wherein said floating plateincludes a plurality of leaf springs that contact inner surfaces of saidfixed stage frame for minimizing backlash.
 11. The light engine of claim10, wherein said motors include threaded motor shafts extending throughsaid fixed stage frame and into threaded holes in said floating plate.12. The light engine of claim 11, wherein said motor shafts include apair of differential threads, a first one of said threads being threadedinto said floating plate and the other one of said threads being matedwith a thread compensator that pulls said fixed stage frame against saidleaf springs.
 13. The light engine of claim 12, wherein said first andsaid other one of said threads are of the same thread type but havedifferent pitches, such that each rotation of each of said shaftsresults in adjustment of the relative position between the fixed stageframe and floating plate according to the difference of said pitches.